1. Field of the Invention
This invention relates to microneedles. More specifically, it relates to microneedles having sharpened sidewall tips and the corresponding method of fabricating them.
2. Description of the Prior Art
Many systems involve the transfer of physical entities through the skin, including needle puncture, electroporation, and removal of the stratum corneum through gels or tapes.
These methods, however, are all fairly intrusive. A minimally invasive method for sampling or delivering biological fluids is essential to autonomous therapy systems. Microneedles can achieve this task with significantly less trauma since their sharp tips and short lengths reduce the odds of encountering a nerve. They either do not penetrate deep enough to reach the underlying nerves, thus being totally painless, or they just graze the tips of nerves, causing sensation but reducing pain nevertheless.
An important design parameter in the fabrication of microneedles is the compromise between structural rigidity and ease of penetration. Due to the elasticity of skin, a considerable amount of deformation takes place around the insertion site, significantly reducing the insertion depth of microneedles. To mitigate the undesirable effects of skin deformation, the pressure applied by the needle tips needs to be increased. Insertion pressure can be increased by raising the applied force or by increasing the needle sharpness. An increase in applied force, however, intensifies the strain on the microneedles and may cause undue breakage. It may also magnify the patient's discomfort. Vibratory actuation has been suggested to decrease the force required for insertion. However, this requires the use of a vibratory actuator, which may not be possible for all applications, or might complicate the design requirements.
Permeation of microneedles can also be improved by increasing their sharpness. Sharpness is often achieved at the expense of structural rigidity. The needles must be capable of tolerating stresses related to the non-uniformity of the skin contour, inadvertent slippage during insertion or removal, and reasonable human movements during penetration. Microneedles with extremely sharp tips but thin needle bodies have been employed with successful penetration into the skin. However, structural damage after insertion was reported due to compromised sidewall thickness. Various approaches have used pyramidal or conical shaped structures to decrease the insertion area. Such designs achieve sharpening of the needle tip by gradually reducing the diameter of the lumen from the base to the tip. Reducing the lumen diameter, in general, reduces needle strength causing the top portions of the needles to be prone to breakage. Some other approaches have used a side-opened ‘spearhead’ structure at the tip of the needles. Apart from being extremely sharp, such designs also solve the issue of microneedle clogging; unfortunately, however, they also increase the fragility of the needles and are relatively complex to fabricate. Making the tip of the needles biodegradable is another approach having similar limitations. Yet another approach is fabricating needles with the entire length beveled. Such needles are relatively robust, however. Similarly, needles having only their tips beveled have been fabricated, but possess relatively limited sharpness.
Accordingly, what is needed in the art is a microneedle that easily penetrates the skin without increased applied force due to the microneedle's sharpness and rigidity. What is also needed is a method of fabricating said microneedle. However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the limitations of the art could be overcome.